Mechanisms of sensory transduction and its modification by motor control will be studied in mammalian muscle spindles, both in situ and in isolated preparations. The membrane properties and contractile activation of intrafusal muscle fibers as well as their passive mechanical properties will be investigated using intracellular recording and stimulation and microscopic measurement of sarcomere length changes. The roles of the 2 types of nuclear bag fiber will be tested by activation of these fibers individually while observing the effect on the primary ending response to stretch. Possible stretch excitation in bag fibers will be studied. The conductance change in the sensory terminals in response to stretch will also be investigated as will the characteristics of impulse initiation in the primary ending.